The present invention relates to coal-water mixed fuel obtained by mixing coals and water and the producing method thereof. More particularly, the present invention relates to high-density coal-water mixed fuel which can maintain the good fluidity with a high density and the producing method thereof.
One method for utilizing coals has been recently proposed. That is, the coals are pulverized and mixed with a small amount of water for slurrying with a high density and pasting in order to enable the transportation using a pipeline or the like. The product obtained by such a method is referred to as the high-density coal-water mixture or slurry (this will be abbreviated as CWM hereinafter) or the high-density coal-water paste (this will be also abbreviated as CWP hereinafter).
In case of the CWM, the density of the coal is increased to 65 through 70 wt % by adjusting the coal particle size distribution to provide the fluidity, and the coal is directly burned in an ordinary boiler without dehydration. Meanwhile, in case of the CWP, the coal particle size distribution is adjusted so that the particle size can be equal to or less than 6 mm which is slightly larger than that in the CWM, and water is added to the coals together with the desulfurizing agent to provide the density of 70 through 80 wt % to give the fluidity. The CWP is then pushed out from the pipeline into a fluidized bed combustion boiler by a pump and burned without making any change. In order to carry out these processes, the water density is decreased as low as 30 through 35 wt % and the sufficient fluidity is required in the CWM or CWP.
Although the method for producing the CWM or CWP has been already commercialized in the wet manufacturing method utilizing the wet grinding, the stronger grinding power is required when carrying out the wet grinding, which increases the manufacturing cost. Development of the dry manufacturing method utilizing the dry grinding with the reduced grinding power is thus desired. In the dry manufacturing method, drying the pulverized coals during pulverization provides the strong water repellency and makes the slurrying difficult. Therefore, in the conventional CWM or CWP production, in order to facilitate flow using the pipeline by slurrying the pulverized coals having the strong water repellency, it is required to add 0.1 through 1 wt % of the dispersant having the general surface active agent as a main component which may be substituted any other material depending on properties of the surface active agent when producing the CWM having a high density of, e.g., 65 through 70%. This improves the wettability of the pulverized coals and prevent aggregation of the pulverized coals in water. Of course, it is similarly necessary to add a large amount of the surface active agent in the wet producing method in order to improve the wettability of the pulverized coals and prevent aggregation of the pulverized coals.
However, in the CWM or CWP described above, the cost of the dispersant per unit is relatively high, and hence the cost of the dispersant accounts for about 20 to 40% of the cost of the CWM or CWP, even the amount of the dispersant accounts for 0.1 through 1 wt % of the amount of the CWM or CWP.
Various kinds of dispersant have been proposed for reducing the cost of the dispersant. For example, although the dispersant which has a high efficiency and whose amount can be reduced has been developed, this type of dispersant disadvantageously increases the cost per unit. Further, the inexpensive dispersant has been also developed, but an amount of this dispersant to be added must be increased. Thus, reduction in the cost of the dispersant is difficult, and hence the cost of the CWM or CWP can not be lowered.
In addition, the fluidity of the CWM or CWP depends on how the particles fill. The middle-sized particles enter into a gap between the large particles, and the small particles enter into a gap between the middle-sized particles. Further, the superfine particles enter a gap between the small particles, and water enters a gap between the superfine particles. This small amount of water generates the fluidity, and the superfine particles which exist around the relatively large particles having the size not less than a few xcexcm and have the size of approximately 1 xcexcm serve as the lubricant, thus assuring the fluidity.
However, in the conventional CWM or CWP producing method in the dry manufacturing method, the pulverized coal obtained by the dry pulverization has an unspecified substantially-polyhedral angular shape, and a large gap is then made between the particles. The gap is not filled even though the regularly-generated amount of superfine particles is introduced, thereby making realization of the high density of the CWM difficult. Furthermore, even if realization of the high density of the CWM is possible, the lack of the superfine particles causes the relatively-large coal particles (a few xcexcm or more) to come into contact with each other without the superfine particles, thereby making the enhancement of the fluidity difficult.
In order to realize the high density and enhance the fluidity of the CWM or CWP, there is considered a method such as that a large amount of coal particles having the size of approximately 1 xcexcm which are referred to as superfine particles is prepared and mixed and such particles are provided between the large coal particles.
However, in the above-described CWM or CWP producing method, since a great amount of superfine particles which are relatively difficult to be pulverized is required, and the mass production is hard to be effected, which actually leads to difficulty in reduction in the manufacturing cost. It is to be noted that the CWM and CWP are generally referred to as the high-density coal-water mixed fuel in this specification and the high-density coal-water mixed fuel includes the high-density coal-water paste as well as the high-density coal-water slurry unless it is specified.
It is an object of the present invention to provide the high-density coal-water mixed fuel which can maintain the fluidity even if the density is increased. More particularly, an object of the present invention is to provide the high-density coal-water mixed fuel and the producing method thereof with which the cost for the dispersant can be reduced. It is another object of the present invention to provide the inexpensive high-density coal-water mixed fuel producing method which can mass-produce the CWM or CWP by the dry grinding without mixing a large amount of superfine particles.
Various kinds of study for achieving these aims caused the present inventors to perceive that the high-density coal-water mixed fuel has the coal particles dispersed in the water and contains a large amount of particles having the size of 1 xcexcm or less and the fuel hence corresponds to the colloid dispersed system or ranges from the coarse particle dispersed system to the colloid dispersed system. Therefore, it is enough to prevent the dispersed particles from being connected with each other in order to maintain the colloid stable and, as one of methods for attaining such prevention, the inventors considered to utilize the so-called the protective effect of the colloid. That is, the affinity between the dispersion medium and the dispersoid is utilized and the hydrophilic colloid is adsorbed to the surface of the coal particles which are the hydrophobic colloidal particles to demonstrate the characteristic as if the coal particles are the hydrophilic colloid, thereby increasing the stability. The pulverized coal slurry generated by mixing the pulverized coal, the water and the hydrophilic colloid, however, has the increased viscosity and the deteriorated fluidity. It was discovered that mixing the dispersant whose amount is smaller than that usually added to the slurry can reduce the viscosity to provide the excellent fluidity. This phenomenon is considered to be caused for the following reasons.
(1) Gelation and Solation by the Protective Colloid
The hydrophilic colloid is adsorbed to the surface of the pulverized coal particles which are the hydrophobic colloid particles and the surface of the pulverized coal particles is covered with the hydrophilic colloid to achieve hydrophilicity. This causes the hydrophilic colloid to show the protective action with respect to the pulverized coals as the protective colloid. The pulverized coal particles which have adsorbed the protective colloid are subjected to the secondary bond by the ionic bond or the like through the multiply charged ion such as metallic ion solved out from the pulverized coal particles and the reversible pulverized coal gel is generated. It can be assumed that such process increases the viscosity of the slurry and degrades the fluidity of the same.
The secondary bond achieved between the pulverized coal particles is destroyed by mixing the dispersant into the slurry and the pulverized coal gel is returned to the sol. The pulverized coal particle keeps its hydrophilicity and becomes stable by the protective action of the protective colloid without aggregation. As a result, it can be assumed that the high-density coal-water mixed fuel having the sufficient fluidity can be provided.
In this case, an amount of the dispersant to be added is enough if it can destroy the secondary bond between the pulverized coal particles, and hence the amount of the dispersant to be added can be reduced as compared with the case when only using the dispersant which is used for preventing aggregation of the pulverized coal particle without adding the hydrophilic colloid.
(2) Aggregation of the Fine Particles Caused by the Electrolyte and Dispersion Caused Owing to Antagonism of the Ion
Since the pulverized coals are fine particles and have electric charges, the ion having the reverse sign (counter ion) is attracted therearound, which constitutes the double structure called the electrical double layer. The pulverized coal particles are usually dispersed colloidally by the electrical repulsion of the counter ions. However, if the electrolyte is given by addition of the hydrophilic colloid, the counter ions are pushed against the surface of the particles, thereby reducing the thickness of the fine electrical double layer. It can be considered that decrease in the distance between the particles causes the respective pulverized coal particles to enter the range of attraction between the particles and to aggregate.
Mixing the dispersant into the slurry adds the electrolyte that is different from the above-described electrolyte. Two or more kinds of electrolyte are added to the pulverized coal particle and the aggregating force of the pulverized coal particle is suppressed by the antagonism of the ion. It is considered that this process can provide the high-density coal-water mixed fuel having the sufficient fluidity.
In this case, since an amount of the dispersant to be added is enough if it can provoke the antagonism of the ion with respect to the pulverized coal particles, the amount of the dispersant to be added can be reduced as compared with the case where only the dispersant is used to prevent aggregation of the pulverized coal particles without adding the hydrophilic colloid.
(3) Aggregation of the Fine Particles Caused Due to the High Polymer Material and Dispersion of the Pulverized Coals Caused by the Bimolecular Layer Adsorption of the Dispersant
Since the hydrophilic colloid is a water soluble high polymer substance and has many hydrogen bonding groups, the hydrophilic colloid is adsorbed to the pulverized coal particles by the hydrogen bonding groups irrespective of the electricity or the ion. If a small amount of the high polymer is adsorbed to the pulverized coal particles, it is not evenly adsorbed but sparsely adsorbed to the surface of the particle. A part of the high polymer adsorbed to the particle is, therefore, adsorbed to the unoccupied area on the surface of a different particle, and one high polymer is hence bonded to two or more particles. It can be assumed that the pulverized coal particles are aggregated by such action. This is a phenomenon called xe2x80x9cthe cross linking aggregationxe2x80x9d.
Mixing the dispersant into the slurry causes the ion of the dispersant to be subjected to the bimolecular layer adsorption to the unoccupied area on the surface of the particle. The entire pulverized coal particles thus have the electric charges and are dispersed. As a result, it is assumed that the high-density coal-water mixed fuel having the sufficient fluidity can be obtained.
By mixing the dispersant into the pulverized coal particles which have been subjected to the cross linking aggregation, the high polymer bonded to one pulverized coal particle is also bonded to the unoccupied area on the surface of the same particle. Multiple high polymers are entangled around the respective particles and become the high polymer like a knitting ball to cover the entire surface of the pulverized coal particle. The high polymers for bonding the particles are reduced and the respective particles repulse. It is considered that such a phenomenon can provide the high-density coal-water mixed fuel having the sufficient fluidity.
In this case, an amount of the dispersant to be added is enough if the dispersant itself or the knitting-ball-like high polymer can cover the unoccupied area of the pulverized coal particle, and hence the amount of the dispersant to be added can be reduced as compared with the case where only the dispersant is used to prevent the pulverized coal particle from aggregating without adding the hydrophilic colloid.
Incidentally, it can be considered that not only one of the above-described phenomena but also the respective phenomena can be simultaneously occur while they are associated with each other. Also, it can be assumed that the fluidity can be similarly obtained even though an amount of the dispersant to be added is greatly reduced for any reason other than the above-described reasons.
On the basis of the above-mentioned knowledge, the high-density coal-water mixed fuel obtained by mixing the pulverized coal, the water and the dispersant includes the hydrophilic colloid that causes the protective effect with respect to the pulverized coals according to the present invention. The hydrophilic colloid causing the protective effect to the pulverized coals is added and mixed in the high-density coal-water mixed fuel when manufacturing the high-density coal-water mixed fuel by mixing the water, the dispersant and the pulverized coals obtained by grinding the coal so as to provide a predetermined particle size distribution.
Therefore, an amount of the dispersant to be added is enough if it can destroy the secondary bond of the pulverized coal particles, if it can cause the antagonism of the ion with respect to the pulverized coal particles, or if it can cover the unoccupied area on the surface of the pulverized coal particle by the dispersant itself or the knitting-ball-like high polymer, and hence the amount of the dispersant to be added can be greatly reduced as compared with the case where only the dispersant is used to prevent aggregation of the pulverized coal particles as in the prior art. For example, when producing the CWM whose density is 70%, the fluidity of the CWM was not lost even though an amount of the surface active agent was reduced to approximately ⅓ of the usual amount as apparent from the measured data shown in FIG. 14. In addition, as apparent from the measured data in FIG. 14, it was possible to produce the CWM having the relationship between the density and the viscosity of the coal substantially unchanged even though the amount of the dispersant to be added is reduced to xc2xd of the usual amount by adding the hydrophilic colloid.
The cost of the high-density coal-water mixed fuel can be, therefore, reduced by lowering the cost of the dispersant by decreasing the amount of the dispersant to be added while maintaining the fluidity of the CWM unchanged.
The existing manufacturing equipment for the high-density coal-water mixed fuel can be utilized as it is because the hydrophilic colloid is only added, thus substantially requiring no increase of the equipment.
Here, the amount of the hydrophilic colloid to be added slightly reduced the amount of the surface active agent to be added if the hydrophilic colloid demonstrating the protective effect to the pulverized coals and the surface active agent were simultaneously added, but the effect was not enough as the above-mentioned effect. Further, addition of the surface active agent followed by that of the protective colloid did not lead to reduction in the amount of the surface active agent used. In production of the high-density coal-water mixed fuel, it is preferable to add the hydrophilic colloid in a mixture of the pulverized coals and the water and thereafter add the dispersant. In such a case, the gel type pulverized coal slurry is generated by adding and mixing the hydrophilic colloid in the mixture of the pulverized coals and the water. The gel type pulverized coal slurry becomes the sol by adding and mixing the dispersant in this slurry. The reason why this phenomenon occurs is described above. This process enables production of the high-density coal-water mixed fuel in which the amount of the dispersant is largely reduced. For example, as apparent from the measured data shown in FIG. 14, the amount of the dispersant can be reduced to xc2xd to xc2xc of the conventional amount of the same by adding 1 ppm of the hydrophilic colloid prior to the dispersant in order to obtain the high-density coal-water mixed fuel having the fluidity equivalent to that of the conventional high-density coal-water mixed fuel having the dispersant of, e.g., 0.4 wt % without adding the hydrophilic colloid.
Moreover, the amount of the hydrophilic colloid may be enough and small when the hydrophilic colloid is adsorbed to the coal fine particle which is the hydrophobic colloid particle and demonstrates the protective effect for making the surface of the coal fine particle hydrophilic, or preferably it may be smaller than 1 wt % of the entire high-density coal-water mixed fuel and larger than that which can provoke the reciprocal aggregation with the pulverized coal, or more preferably it may be ranged between the ppm order to the 10xe2x88x923 ppt order of the entire fuel, or most preferably it may be ranged between the ppt order to the ppb order of the same. Since increase in the amount of the hydrophilic colloid to be added causes bonding between the pulverized coals to be stronger by the high gelatination, the amount of the dispersant to be added must be increased in order to destroy this bonding, thereby deteriorating the reduction effect of the dispersant. On the contrary, if the amount of the hydrophilic colloid to be added is too small, such an amount causes the sensitizing effect resulting in the unstable hydrophobic colloid. Specifically, as apparent from the measured data shown in FIG. 13, when the amount of the colloid to be added to the water exceeds 10 ppm in order to obtain the CWM having the density of 70%, the fluidity is degraded. If the amount of the hydrophilic colloid to be added is smaller than 10xe2x88x924 ppt, the fluidity is also deteriorated.
In addition, the method for producing the high-density coal-water mixed fuel according to the present invention, the coal is powdered by using a mill to obtain the pulverized coal having the size substantially smaller than a predetermined value, angles of the pulverized coals are removed to provide a spherical shape and to generate the superfine particles by abrading these coals together by using a spheroidizing apparatus for pushing and rubbing the pulverized coals, thereby the high-density coal-water mixed fuel is produced.
The pulverized coals obtained by grinding the coal by a mill correspond to the fine particles most of which have a particle size lower than a predetermined value, or more particularly, a particle size equal to or less than 100 xcexcm, and this particle is an undefined angular polyhedron which is relatively large for the high-density coal-water mixed fuel, as shown in FIGS. 5(A) and 6. Further, in regard of the particle size distribution (mass basis), the particle size equal to or less than 100 xcexcm accounts for approximately 93%; the particle size equal to or less than 10 xcexcm, approximately 15%; and the particle size equal to or less than 1 xcexcm, less than 1%, as shown by circles in FIG. 12. The fine particle component having the particle size equal to or less than 10 xcexcm is lacking for obtaining the CWM.
However, the pulverized coals are pushed and rubbed in the spheroidizing apparatus, and these coals are ground when they are rubbed together. As shown in FIGS. 5(B) and 7, the pulverized coal loses its angles and is spheroidized to reduce the surface area thereof. Also, the removed angle becomes a superfine particle having the size equal to or less than 1 xcexcm. Therefore, as to the particle size distribution (mass basis), the particle size equal to or less than 100 xcexcm accounts for approximately 100%; the particle size equal to or less than 10 xcexcm, approximately 45%; and the particle size equal to or less than 1 xcexcm, approximately 17%, as shown by black circles in FIG. 12. These satisfy the values required as the CWM. It is possible to obtain the CWM in which a gap between the respective spheroidized pulverized coal particles is filled with the superfine particles. Further, spheroidization of the pulverized coal according to the present invention can be applied to manufacture of the COM (coal-oil mixture).
In other words, when the pulverized coal loses its angles and is spheroidized to reduce the surface area, an amount of the superfine particles required for filling a gap between the respective pulverized coals is decreased. A part that is apt to be scraped off is removed and the body particle is spheroidized, but the original particle size can not be extremely reduced, thus generating the superfine particle. On the other hand, the angle scraped off becomes a superfine particle and fills a gap between the respective spheroidized pulverized coals. A sufficient amount of the superfine particles, therefore, fills the gap between the pulverized coals. This enables adjustment to realize the wide particle size distribution required for the high-density coal-water mixed fuel to obtain the fluidity, i.e., the particle size distribution which enables easy generation of relatively-large spheroidized particles to superfine spheroidized particles by removing angles to spheroidize the pulverized coal and is suitable for the high-density coal-water mixed fuel. Water is removed from the gap between the pulverized coal particles to obtain the CWM or CWP having a high density. Since the superfine particles attached to the surface of the pulverized coal in the covering manner to cause the lubricating effect, the CWM or CWP having the high fluidity can be obtained.
It is apparent from the measured data shown in FIG. 15 that the fluidity of the spheroidized CWM (shown by black triangles) is higher than that of the non-spheroidized CWM (shown by white triangles). Further, production of the CWM or CWP can be facilitated to reduce the manufacturing cost because a large amount of the superfine particles does not have to be mixed. Also, according to the present invention, since pulverizing power is further minimized, the existing manufacturing equipment can be utilized without making any change, and an increase of the equipment is substantially unnecessary.
Moreover, according to the high-density coal-water mixed fuel producing method, first and second members having a small gap between the opposed surfaces thereof are provided in a spheroidizing apparatus; the first and second members are capable of relative motion with the gap between their opposed surfaces being substantially fixed; and the pulverized coals held between the opposed surfaces are pushed and rubbed to be abraded together and the pulverized coal is spheroidized by removing its angles to produce superfine particles. In this case, the wet pulverized coals are pushed and rubbed to be abraded together by the opposed surfaces of the first and second members performing the relative motion so that angles of the pulverized coal are removed to facilitate easy spheroidization. In addition, the removed angles become superfine particles and are separated from the pulverized coal. The spheroidizing apparatus can be inexpensively obtained to reduce the producing cost for the CWM or CWP.